The present invention relates generally to motion control systems, and, more particularly, to a circuit for inhibiting motor protection during motor start-up.
This section of this document is intended to introduce various aspects of art that may be related to various aspects of the present invention described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the present invention. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art.
Induction motors have broad application in industry. An induction motor system typically includes a drive or controller, a motor overload protection device, and an induction motor. The controller may be as simple as an electromechanical starter (i.e., contactor) that simply links the supply lines to the motor terminals through switching contacts without affecting the supply power waveform, or may include a power conversion configuration that generally receives power via supply lines and converts the received power into a form that can be provided to the motor thereby causing a motor rotor to rotate at variable speeds or reduced power. The conversion configuration typically includes a plurality of semiconductor switching devices that link the supply lines to motor terminals and, based on switch turn on and turn off cycles, provide power to the motor phases linked thereto. The motor overload protection device may be required by local electric codes to provide minimal protection against the motor overheating due to an extended period of excess load current from conditions such as locked rotor or excessive motor loading which could lead to a fire.
Typically, a motor system includes at least motor protection circuitry that senses a motor overload condition due to a locked rotor condition or excessive load and interrupts current to the motor to prevent overheating, which could damage the equipment or endanger personnel. Generally, the protection circuitry senses the level of current versus time to determine when to interrupt the current to the motor. Often, motors may have additional protection circuitry that senses a locked rotor or jammed condition of the motor and shuts down the motor to prevent overheating, which could damage the equipment or endanger personnel. Generally, the jam protection circuitry senses the level of the current as compared to the full load amp rating (FLA) of the motor and shuts down the motor if the measured current exceeds the FLA by a predetermined amount (e.g., 400% FLA). The predetermined threshold is commonly referred to as the jam current.
During motor startup, the current required to start the rotation of the motor, commonly referred to as the inrush current, is typically much higher than the FLA rating. For example, the peak inrush current may be from 400% to 1000% of the FLA. For a typical motor, the peak inrush current may be about 600% of the FLA. The duration of the inrush current may vary widely, e.g., from 1 second to 30 seconds.
If the motor protection circuitry is always active, the motor may trip due to the inrush current, if the inrush current exceeds the jam current. One solution for addressing the inrush current interval is to disable the protection circuitry during motor startup to avoid inadvertent motor trips. Typically a startup timer is employed to inhibit the motor protection circuitry for a predetermined time interval, such as 30 seconds, after a motor startup. The inhibit timer begins when motor current is sensed or when a contactor feeding the motor is energized. The timer interval is set to a value that conservatively exceeds the inrush current. The timer may be integrated into a protection device or may be an external device that is wired into the protection circuit.
One limitation of the startup solution is that protection may be inhibited for an unnecessarily long time. During the time interval after the inrush current subsides, but before the inhibit timer elapses, a fault condition may arise that is not addressed immediately.
Another technique for providing motor protection involves employing a microprocessor based protection device that monitors the inrush current profile and inhibits the motor protection using a firmware algorithm. The microprocessor/firmware protection device increases the cost and complexity of the drive system and protection circuitry.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.